Salt-concentration dependence of the structure and form factors for the wormlike micelle made from a dual surfactant in aqueous solutions

Abstract

Small-angle X-ray scattering (SAXS) from dual-surfactant aqueous solutions made from sodium lauryl ether sulfate and coconut fatty acid amido propyl betaine was systematically measured as a function of the net sodium cation concentration, [Na + ]*, and the surfactant concentration, C D . The SAXS intensity [I(q)] was normalized to C D and the resultant I(q)/C D was extrapolated to C D = 0 to give a form factor P(q) for each [Na + ]* [where q = 4 sin( /2)/ is the magnitude of the scattering vector, is the wavelength and 2 is the scattering angle]. The low-q behaviour of P(q) was consistent with long rigid cylinders. The middle-and high-q profiles fitted well with a core-shell cylinder model for all [Na + ]*. The core and total radii (R c and R s ) did not depend on [Na + ]* at all: R c = 1.2 AE 0.05 and R s = 3.1 AE 0.05 nm for [Na + ]* = 0.42-1.5 mol l À1 , indicating that the salt concentration changes did not induce any structural changes and reassembling of the surfactants comprising the micelles. This fact is in contrast to the rheological behaviour where the relaxation mode strongly depends on [Na + ]*. The structure factor [S(q)] was obtained by dividing I(q)/C D by P(q) for each C D and the mean distance (d m ) between the micelles was obtained from the first maximum of S(q) versus q plots. The d m value decreased with increasing C D and [Na + ]*, which is in good agreement with the theoretical prediction and experimental results for charged wormlike micelle solutions.

Revealing New Structural Insights from Surfactant Micelles through DLS, Microrheology and Raman Spectroscopy

“…This is indicative of a significant change in size of the micelles compared to just the SLES or CapB. The overall surfactant concentrations utilized in this study was 450 mM, which is far above the CMC and the sphere-to-rod transition concentrations reported for this system [ 21 , 22 ] (albeit at different SLES/CapB ratios). For our purposes, the weight percentage ratio of 14:2 for the surfactants was chosen due to the mixture remaining homogeneous at high salt concentrations.…”

“…This maybe indicative of more rodlike or cylindrical micelles forming under these conditions. Betaines have been seen to form wormlike micelles in the presence of salts [ 2 , 3 , 6 , 9 , 12 , 13 , 14 , 17 , 21 , 22 , 23 , 24 , 25 ]. The samples containing both SLES/CapB (14% w/w and 2% w/w, respectively) exhibited increasing viscosity and non-Newtonian (shear-thinning) behavior as the NaCl concentration was increased to 250 mM.…”

“…Although a few studies have characterized the formation of wormlike micelles formed in sodium dodecyl sulfate (SDS)/CapB system [ 21 ], to our knowledge, despite its commercial importance, there has been little characterization of the SLES/CapB surfactant mixture [ 22 ]. These limited scattering studies [ 21 , 22 ] on this system have however confirmed the transition to cylindrical and then wormlike micelles for this system at relatively low total surfactant concentrations. To the best of our knowledge, a systematic study on the effect of salt concentration on the evolution of the microstructure/rheology has not, however, been undertaken.…”

Scattering from polymer-like micelles

“…Since the SANS is measured over a limited q-range, the details of the structure are difficult to ascertain, but the formation of elongated micelles in a system of surfactant and counterion of like charge is worth further study [9]. Mixtures of anionic and zwitterionic surfactants show viscoelastic behavior, and SAXS provides a measure of the cross-sectional structure and composition but the q-range is not wide enough to determine lengths or intermicellar interactions [10]. A SANS study of nonionic surfactant doped with a cationic surfactant shows the impact of doping on the rigidity and size of elongated micelles.…”

Micellar Surfactant Association in the Presence of a Glucoside-based Amphiphile Detected via High-Throughput Small Angle X-ray Scattering

“…For example, sodium lauryl ether sulfate (SLES) and CAPB were found to form long rod micelles in the presence of salt, with well-defined core-shell cylindrical shape and a measurable variation in diameter, micellar spacing, and radius of gyration with different surfactant compositions [2]. A double layered cylinder, wormlike micelle phase in SLES-CAPB had been similarly characterized as a function of salt concentration using SAXS [3]. These studies are part of a larger body of literature that demonstrates the SAXS measurement and analysis techniques applied to surfactant systems.…”